Selectable bobbin thread tensions

ABSTRACT

A method and apparatus for obtaining a variety of selectable bobbin thread tensions in a lockstitch sewing machine wherein bobbin thread tension is achieved by resisting bobbin rotation by pressing a bobbin hold down arm against an annular protuberance on a flange of the bobbin and concentric with the center of rotation of the bobbin, replacing the bobbin with one similarly constructed but having its protuberance at a suitable different radius when a change in bobbin tension is required.

[451 Nov. 7, 1972 United States Patent Davidson et al.

3,467,068 11/1969 Preston..................l12/181X [54] SELECTABLEBOBBIN THREAD TENSIONS [72] Inventors: Donald R. Davidson, Parsippany;

Primary Examiner-H. Hampton Hunter Edmund Wilson, I, Dover bothAttorney-Marshall J. Breen et al. of NJ.

[73] Assignee: The Singer Company, New York,

f o v t e n a V a 8 .m .m a am o r 0 Rf TS Em a m P P a d n a d o h t e1 m DA selectable bobbin thread tensions in a lockstitch sew- [22] led:July 1971 ing machine wherein bobbin thread tension is [21] Appl. No.:163,840 achieved by resisting bobbin rotation by pressing a bobbin holddown arm against an annular protube- 44 2 8 25 727 15 2 2b 4 45 2 20 i 1w m 2 U m1 1 9 n 2 4 81 .1 w M 11 1 mmm m m m .L C te I ll U.mF 1].]2.18 555 [[ft 3 Claims, 5 Drawing Figures [56] References Cited UNITEDSTATES PATENTS 875,603 12/1907 Parkes.......................l12/229PA'IENTEIINHV H912 3,702,102

INCREASING BOBBIN THREAD TENSION SS; MEAN DIAMETER OF RING Y )1 6M.)

"fi /M )WCOM ATTORNEY Fig. 5 I

- INVENTOR. I Donald R. Doyidson v BY Edmund B. W||son1II SELECTABLEBOBBIN THREAD TENSIONS SUMMARY OF THE INVENTION This invention relatesto a method of obtaining bobbin thread tension which will enable a sewerto readily select the most appropriate bobbin thread tension for thesewing operation to be performed. Diverse sewing operations, such aszigzag sewing of sheer fabrics without pucker, to sewing with elastomerbobbin thread at maximum tension to obtain the maximum shirring effectcan be accomplished on one machine by utilizing the apparatus of thisinvention.

It is known to provide for lower thread tension of the sewing machine byrestricting the rotation of the bobbin. This is accomplished byutilizing various forms of spring members such as Bellville washers,leaf springs and coil springs alone or in combination with other membersto bear against the rotating bobbin with a pressure sufficient to limitits ready rotation and thereby to apply some tension to removal of thelower thread. It is also generally known to provide means in conjunctionwith the spring members to provide some adjustability for the bobbinthread tension. These generally only provide for limited degrees ofadjustability or are so difficult to calibrate as to place their usebeyond the capabilities of the greater majority of home sewers. What islacking in the prior art is a bobbin thread tension selecting meanswhich may be readily accomplished even by an inexperienced sewer. It is,therefore, an object of this invention to provide a means for readilyselecting bobbin thread tension.

It is a further object of this invention to provide a means for readilyselecting bobbin thread tension which may be readily and accuratelyreproduced.

It is a still further object of this invention to provide for thatselectable bobbin thread tension which will be optimum for particularsewing operations to be performed.

Yet another object of the invention is to provide for this selectabilityof bobbin thread tension in a most economical manner.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a vertical axisrotating hook of a conventional lockstitch sewing machine showing thebobbin and bobbin case of the invention in place therein,

FIG. 2 is an enlarged cross sectional view of the bobbin showing theraised concentric friction ring in solid lines in one location and indotted lines showing the friction ring in two extreme locations.

FIG. 3 is a vertical cross-sectional view taken substantially along line33 of FIG. 1,

FIG. 4 is a graph depicting the variation in the bobbin thread tensionresulting from a variation in the mean diameter of the friction ring,and

FIG. 5 is a plan view of a bobbin case used in FIG. 1 showing amodification of the invention. I

Reference to FIGS. 1 and 3 discloses a vertical axis rotary hook of alockstitch sewing machine which hook rotates in a counterclockwisedirection as viewed in FIG. 1. The hook 10 is formed with a loop seizingbeak 11 which when cooperating with a thread-carrying needle will pickup a needle thread loop and cast it about a supply of bobbin thread in awell known manner. On the inner diameter of the hook 10 there is a isupporting flange or bearing rib 12 on which is supported a bobbin casedenoted generally as 14. The bobbin case 14 is constrained againstrotation with the rotary hook 10 by a position finger 15 attached to thesewing machine frame and also by a cushion spring'16 attached by screw17 to a cushion spring bracket 18. The cushion spring bracket is carriedby the sewing machine frame in a fashion which allows lateral shift ofthe bracket 18 over the top of the hook and against the bobbin caseunder the influence of an eccentric stud 19, thereby to adjust thethread clearances between an abutment 20 on the bobbin case and theposition finger 15, and between an abutment 21 and the cushion springl6'so that the needle loop may be cast about the bobbin case by the loopseizing beak ll.

The bobbin case 14 supports a bobbin 23 on a bobbin receiving flange 24and prevents sidewise shift of the bobbin by the provision of the innerdiameter 25. The bobbin itself is constructed with a smaller diameterlower flange 26 to allow the insertion of thebobbin 23 into the bobbincase and the resting of the lower surface 27 of its upper flange 39 onthe flange 24. Fitted into a cavity 28 (FIG. 1) in the bobbin case andpivoted on a pin 29 is a bobbin hold-down lever arm 30. The lever arm 30has a projection 31 beneath the pivot pin 29 acted on by a leaf spring32 which is attached to the bobbin case by a screw 33 and nut 34. Theleaf spring impinges on an edge of the lever arm projection 31 to imparta moment to the lever arm 30. This moment causes the lower surface 35 ofthe lever arm to press against a concentric raised annular ring 36formed on the upper bobbin flange 39. The force exerted by the lowersurface 35 of the lever arm against the raised annular ring 36 pressesthe lower surface 27 of the upper flange 39 against the bobbin receivingflange 24 of the stationary bobbin case and thereby creates frictionalresistance to bobbin rotation on two different annular friction surfaces36 and 27 resulting in bobbin thread tension.

FIG. 2 is a cross-sectional view of the bobbin showing in solid lines araised annular ring 36 of one diameter concentric about the bobbincenter line and also indicating in phantom lines two other raisedannular rings 37 and 38, respectively, of maximum diameter 37 andminimum diameter 38. Regardless of the position of the annular frictionring the moment exerted by the leaf spring 32 on the lower surface 35 ofthe lever arm 30 is a constant value. Utilizing a bobbin with thelargest diameter raised annular friction ring 37 will provide theshortest lever arm distance on the lever arm 30 resulting in thegreatest force exerted on the bobbin by the lever arm. For a givencoefficient of friction between the lever arm 30 and the raised annularring 37 and between the lower surface 27 of the upper flange of thebobbin and the bobbin receiving flange 24 of the bobbin case there willcorrespondingly be generated the largest retarding moment to bobbinrotation which will result in large bobbin thread tension.

On the other hand, utilizing a bobbin having the smallest diameterraised annular ring, as indicated by 38, will provide the longest leverarm distance on the lever arm 30, resulting in the least force exertedon the bobbin by the lever arm. The resulting low frictional force onthe raised annular ring of the bobbin and on the lower surface of theupper flange of the bobbin when combined with the effect of the smallradius of the raised annularring from the bobbin center will produce aminimum bobbin resistance to rotation or minimum bobbin thread tension.

Theeffect of the diameter of the raised annular ring on bobbin threadtension is shown in FIG. 4. The in crease in bobbin tension is notlinear with the increase in the mean diameter of the raised annular ringbecause there are two radii involved, that of the hold-down arm where itimpinges the raised annular ring and that of the. raised annular ringitself.

In FIG. 5 is shown a bobbin case having a modification of the inventionwhich may be used on bobbins having no raised annular friction ringthereon. With this modification tension adjustments may be made withoutchanging bobbins. In this view is shown a bobbin case hold-down arm 45which is supported on a pivot pin 29 and acted upon by a leaf spring 32,as in the construction shown in FIGS. 1, 2 and 3. The holddown arm 45 isformed with scribe marks 46, 47 and 48 at different distances from thepivot pin 29.-A friction shoe 49 slidably affixed to the bobbin casehold-down arm 45 will provide the rubbing surface between a conventionalbobbin and the hold-down arm 45, thereby taking the place of the raisedannular rings 36, 37 and 38 previously described. The scribe marks 46,47 and 48 may be previously calibrated to locate the positions for thefriction shoe which will give rise to bobbin thread tension appropriatefor zigzag stitching and straight sewing as well as for obtainingshirring effects with elastomeric threads. Making bobbin thread tensionadjustment from one type of sewing operation to another simply requiressliding of the friction shoe 49 to the appropriate scribe mark beforeproceeding with the sewing operation.

Having thus set forth the nature of this invention,

what is claimed herein is:

1. In a lockstitch sewing machine wherein bobbin thread tension isachieved by resisting bobbin rotation, selectable bobbin threadtensioning means comprising a bobbin case adapted to support a flangedthread carrying bobbin for rotation therein, means carried on saidbobbin case to retain said bobbin in said bobbin case,

means associated with said retaining means'for applying a rotationresisting force to said bobbin in said bobbin case at selectable radii,whereby rotation of the bobbin may be impeded a selectable amount toproduce a selectable bobbin thread tension.

2. Selectable bobbin thread tensioning means as set forth in claim 1 inwhich said means to retain said bobbin includes a lever arm carried bythe bobbin case, and a resilient member to urge said lever arm againstsaid bobbin; said bobbin being one of a set of bobbins each having aflange formed with a raised annular protuberance concentric to thebobbin axis of rotation and at differing radii for each bobbin in saidset, said lever arm and said resilient member cooperating tofrictionally engage said raised annular protuberance on said flange ofany selected bobbin of the set of bobbins, whereby said differing radiifor said raised annular protuberances will result in differing bobbinthread tension.

3. Selectable bobbin thread tensioning means as set forth in claim 1 inwhich said means to retain said bobbin includes a lever arm carried bysaid bobbin case,

and a r sil ient member to urge said lever arm against said bo bin; andsaid means for applying a norma force to said bobbin in said bobbin caseat selectable radii includes a friction shoe means slidable on saidlever arm to allow for selecting radius of force application to thebobbin whereby selectable resistance to bobbin rotation and therebybobbin thread tension may be obtained.

1. In a lockstitch sewing machine wherein bobbin thread tension isachieved by resisting bobbin rotation, selectable bobbin threadtensioning means comprising a bobbin case adapted to support a flangedthread carrying bobbin for rotation therein, means carried on saidbobbin case to retain said bobbin in said bobbin case, means associatedwith said retaining means for applying a rotation resisting force tosaid bobbin in said bobbin case at selectable radii, whereby rotation ofthe bobbin may be impeded a selectable amount to produce a selectablebobbin thread tension.
 2. Selectable bobbin thread tensioning means asset forth in claim 1 in which said means to retain said bobbin includesa lever arm carried by the bobbin case, and a resilient member to urgesaid lever arm against said bobbin; said bobbin being one of a set ofbobbins each having a flange formed with a raised annular protuberanceconcentric to the bobbin axis of rotation and at differing radii foreach bobbin in said set, said lever arm and said resilient membercooperating to frictionally engage said raised annular protuberance onsaid flange of any selected bobbin of the set of bobbins, whereby saiddiffering radii for said raised annular protuberances will result indiffering bobbin thread tension.
 3. Selectable bobbin thread tensioningmeans as set forth in claim 1 in which said means to retain said bobbinincludes a lever arm carried by said bobbin case, and a resilient memberto urge said lever arm against said bobbin; and said means for applyinga normal force to said bobbin in said bobbin case at selectable radiiincludes a friction shoe means slidable on said lever arm to allow forselecting radius of force application to the bobbin whereby selectableresistance to bobbin rotation and thereby bobbin thread tension may beobtained.